When a person stops breathing, oxygen levels in the blood decrease. This happens again and again with sleep apnea.

MICHAEL TWERY: "And it's constantly exposing us, night after night, to periods of insufficient oxygen. The level of oxygen in our blood will actually decrease to levels that would be considered a medical emergency."

Dr. Twery compares the effect of sleep apnea to racing a car engine for long periods of time.

MICHAEL TWERY: "Our heart becomes overworked and we become more vulnerable to heart attack."

And also strokes. Dr. Twery led a study of about nine thousand people who had sleep apnea but no history of heart disease. The researchers followed their progress for nine years.

MICHAEL TWERY: "They found that men can experience up to a three-fold increased risk of stroke, and that risk seemed to be well correlated with the severity of sleep apnea."

In other words, the more severe the sleep apnea, the greater the chance of a stroke.

The next step will involve sleep apnea patients who have already had a stroke or heart attack. Researchers will study whether patients can reduce the risk of a second one with a machine called a CPAP. CPAP is continuous positive airway pressure. It provides a continuous flow of air into the throat and lungs while the person sleeps.

Sleep apnea seems to be more common in men than in women, and it becomes more common as people get older. The most common form is called obstructive sleep apnea. People who have it are often overweight or have it in their family, but it can affect anyone. In children, for example, enlarged tonsils in the throat can interfere with breathing as they sleep.